Details/Summary: Improve support for heading classes/elements

[EricDunsworth]

Fixes layout issues when styling summaries as headings and/or placing headings within them.

Specific changes:

• Add a second set of heading font size variables that represent pixel units (uses a similar naming scheme as #1779)
• Don't disable borders on summaries that use the "h3, "h4", "h5" and "h6" classes (they were disabling the summary's border on collapsed details elements)
• Rename the $details-identation variable to $details-indentation (typo fix)

Notes:

• Complements wet-boew/wet-boew#9045
• Fixes #1750

[EricDunsworth]

@GormFrank This will fail to build until wet-boew/wet-boew#9045 has landed and GCWeb is correctly referencing it (since both WET and GCWeb are currently using the typo-ed $details-identation variable).

Btw I'd also recommend merging #1779 prior to this. This PR might need to be rebased afterwards... Neither PR changes the exact same lines, but they change blocks of variables that are situated near one-another. So if git tries to use one of the lines #1779 changed to infer the position of this PR's new variable declarations, it might get treated like a conflict.

Edit: I think I was wrong about the need for rebasing... There's an empty unchanged line between both sets of variable delcarations in this PR and #1779... so merge conflicts probably won't occur.

[duboisp]

ci build fail:

Running "sass:all" (sass) task Fatal error: Undefined variable: "$details-indentation".

[EricDunsworth]

@duboisp wet-boew/wet-boew#9045 and this PR are like two sides of the same coin. You can't successfully build one without the other.

To make this PR build successfully:

1. Merge wet-boew/wet-boew#9045
2. Release a new version of WET
3. Update GCWeb's dependencies to use the new version of WET (will cause GCWeb's normal build to start failing)
4. Rebase this PR and trigger a rebuild of it

The WET PR corrected a typo in the $details-identation variable's name (it's now called $details-indentation). But GCWeb references that variable. So I made the same typo fix in this PR too. If I undo it, GCWeb's CI build will start working... but will break again once the WET PR lands. So my hands are tied.

[EricDunsworth]

I rebased this yesterday and it's currently fully in line with wet-boew/wet-boew#9045... but that PR is still in draft and is probably going to require significant changes that'll affect this one. So I'll mark this as a draft for now.